Structure of R3PCl2 compounds in the solid state and in solution: dependency of structure on R. Crystal structures of trigonal bipyramidal (C6F5)3PCl2, Ph2(C6F5)PCl2 and of ionic Prn3PCl2

Abstract

A number of triorganophosphorus dichloride compounds R₃PCl₂, (R₃ = substituted aryl, mixed aryl–alkyl or triaryl) have been synthesized from diethyl ether solution and characterised by analytical and ³¹P-{¹H} NMR data in CDCl₃ solution. The majority of the compounds are ionic, [R₃PCl]Cl, in CDCl₃ solution, in keeping with analogous species containing the heavier halogens [R₃PX]X (X = Br or I), according to ³¹P-{¹H} NMR studies. In contrast, the compounds R₃PCl₂ [R₃ = (C₆F₅)₃ or (C₆F₅)Ph₂] have a molecular five-co-ordinate trigonal-bipyramidal structure both in CDCl₃ solution and in the solid state. The crystal structures of these two compounds have been determined and represent the only crystallographic studies of trigonal-bipyramidal compounds of stoichiometry R₃PCl₂. The compound (C₆F₅)₃PCl₂ exhibits almost perfect trigonal-bipyramidal geometry, whereas (C₆F₅)Ph₂PCl₂ shows significant distortion. This may be due to the asymmetry of the equatorial groups around the phosphorus atom. Why R₃PCl₂ [R₃ = (C₆F₅)₃ or (C₆F₅)Ph₂] adopt a trigonal-bipyramidal structure is reasoned to be due to the acidity of the parent tertiary phosphines, which favours this geometry for the dihalogen adducts, a phenomenon previously observed for dihalogen adducts of tertiary arsines. The crystal structure of Prⁿ₃PCl₂, the first crystallographically characterised example of an ionic R₃PCl₂ compound which does not contain a solvent molecule, has been found to contain two Prⁿ₃PCl₂ entities. The first consists of an ionic [Prⁿ₃PCl]⁺ unit weakly linked by a long Cl· · ·Cl contact to a Cl^–, d(Cl· · ·Cl) 3.207(3) Å. The second shows a discrete [Prⁿ₃PCl]⁺ cation, the Cl^– anion being associated with δ⁺ H atoms on a [Prⁿ₃PCl]⁺ moiety. This compound was prepared and crystallised from diethyl ether and its relation to the solvated complex [Ph₃PCl· · ·Cl· · ·ClPPh₃]Cl·CH₂Cl₂ is discussed.